1. Field of the Invention
The present invention relates to a DC-AC converter, and more particularly, to a converter dynamically adjusting the frequency of a pulse width modulation signal for increasing switching efficiency.
2. Description of Related Art
With great advance in the techniques of electro-optical and semiconductor devices, flat panel displays, such as liquid crystal displays (LCD), have enjoyed burgeoning development and flourished in recent year. Due to the numerous advantages of the LCD, such as low power consumption, free of radiation, and high space utilization, the LCD has become the main stream in the market. An LCD includes a liquid crystal display panel and a backlight module. The liquid crystal display panel has no capacity of emitting light by itself so that the backlight module is arranged below the liquid crystal display panel to provide the surface light source for the liquid crystal display panel so as to perform the display function.
Generally, a cold cathode fluorescent lamp (CCFL) is often used in the backlight module for providing a backlight. An inverter circuit (or called a DC-AC converter) is needed to generate a driving signal with alternating current (AC) to drive the CCFL. FIG. 1 is a circuit diagram of a conventional DC-AC converter. Referring to FIG. 1, the DC-AC converter 100 includes a switch SW1, a capacitor CA1, a pulse width modulator 110, a transformer 120 and a voltage detector 140. The conducting states (i.e. turn on state or turn off state) of the switch SW1 is controlled by a pulse width modulation (PWM) signal P1. With the change of the conducting states, the capacitor CA1 is charged or discharged via a current path provided by a primary winding of the transformer 120. A secondary winding of the transformer 120 induces a current variation of the primary winding and generates an AC voltage to drive the CCFL 130. Since the luminance of the CCFL 130 is determined according to the amount of current flowing through the CCFL 130, the voltage detector 140 detects a current flowing through the CCFL 130 and converts that current signal into a voltage signal as a feedback signal fb.
When the DC-AC converter 100 operates with a fixed power supply voltage VCC, the pulse width modulator 110 can adjust the frequency of the PWM signal P1 for achieving optimal switching efficiency. Nevertheless, a variation of the power supply voltage VCC, e.g. the power supply voltage VCC becomes higher or lower, may influence the switching efficiency of the switch SW1 if the frequency of the PWM signal P1 is kept invariable. That is to say, the switching efficiency of the switch SW1 gets worse and thereby the power consumption of the DC-AC converter 100 increases. Nowadays, in the design of the backlight module, the factories and stores attempt to solve the said problem for increase the switching efficiency of the DC-AC converter effectively.